Demystifying the 10 principles of the extruder, old plastic granulation must read!

The working mechanism of the screw extruder relies on the pressure and shear force generated by the rotation of the screw, so that the material can be fully plasticized and uniformly mixed and molded by the die. Do you know the principles of plastic extrusion mentioned below?

1. Mechanical principle

Most single screws are right-handed threads, like screws and bolts used in woodworking and machinery. If they look from the back, they are rotating in the opposite direction because they try to spin out the barrel as far as possible. In some twin-screw extruders, the two screws rotate in opposite directions in two cylinders and cross each other, so one must be right-handed and the other must be left-handed.

2, the principle of heat

Thermoplastics are generally used in extrusion production where the material melts upon heating and solidifies upon cooling. Where does the heat of the molten plastic come from? The feed preheating and the barrel/die heater play a big role, the motor input energy and the motor overcome the resistance of the viscous melt. The frictional heat generated in the barrel when turning the screw is all plastic. The most important source of heat during extrusion.

3, the principle of deceleration

In most extruders, the change in screw speed is achieved by adjusting the speed of the motor. The motor typically rotates at full speed of approximately 1750 rpm, but this is too fast for an extruder screw. If it is rotated at such a fast speed, too much frictional heat is generated and the residence time of the plastic is too short to prepare a uniform, well-stirred melt. Typical deceleration ratios range from 10:1 to 20:1.

4, feed as a coolant

Extrusion is the transfer of energy to the unheated plastic to melt it, and the input feed is at a lower temperature than the barrel and screw surfaces in the feed zone. The surface temperature of the barrel in the feed zone is at the melting point of the plastic, cooled by contact with the feed particles, but the heat is retained by the heat transferred back to the hot front end and controlled heating. Even after the current end heat is held by the viscous friction and no barrel heat input is required, the post heater may be required.

5. The material in the feeding area sticks to the cylinder and slides onto the screw.

In order to maximize the amount of solids transported in the smooth barrel feed zone of a single screw extruder, the particles should stick to the barrel and slide onto the screw. If the particles stick to the root of the screw, nothing pulls them down, and the volume of the passage and the amount of solids are reduced. Another reason for poor adhesion to the roots is that the plastic may heat up here and produce gels and similar contaminating particles, or intermittently adhere and break with changes in output speed.

6, the highest cost of materials

In some cases material costs can account for 80% of production costs, more than all other factors. This principle naturally leads to two conclusions: processors should reuse scrap and scrap as much as possible in place of raw materials, and strictly adhere to tolerances as much as possible to avoid deviations from target thickness and product problems.

The energy cost is not very important. The energy required to run an extruder is actually a small part of the total production cost. Because the material cost is very high, the extruder is an effective system. With more energy, the plastic will overheat so that it cannot be processed normally.

7, screw end pressure is very important

This pressure reflects the resistance of all objects downstream of the screw: the filter screen and the contaminated shredder plate, the adapter tube, the fixed stirrer, and the mold itself. It depends not only on the geometry of these components but also on the temperature in the system, which in turn affects resin viscosity and throughput. Measuring temperature is important for safety reasons - if the temperature is too high, the die and mold can explode and cause damage to the machine and nearby people.

8, the output

Output = displacement of the last thread +/- pressure flow and leakage, the displacement of the last thread is called positive flow, depending only on the geometry of the screw, screw speed and melt density. It is regulated by the pressure stream and actually includes a drag effect that reduces the output (indicated by the highest pressure) and any overbiting effect in the feed that increases the output. The leak on the thread may be in either of two directions.

9. Shear rate plays a major role in viscosity

Most plastics have a shear thinning characteristic, that is, the viscosity is lower as the plastic is sheared. For example, some PVCs increase the flow rate by a factor of 10 or more when the thrust is doubled. The LLDPE shear force is not reduced too much, and the flow rate is only increased by a factor of three to four when the thrust is doubled. The reduced shear reduction effect means high viscosity under extrusion conditions, which in turn means more motor power is required.

10. The opposite relationship between the motor and the cylinder

Why is the control effect of the cylinder not always the same as expected? If the cylinder is heated, the viscosity of the material layer at the wall of the cylinder becomes smaller, the motor requires less energy to operate in this smoother cylinder, and the motor current decreases. On the contrary, if the cylinder is cooled, the viscosity of the melt at the wall of the cylinder increases, the motor must be rotated more forcefully, the amperage is increased, and some of the heat removed by the cylinder is returned by the motor.

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